Right, As some of you know i have just bought a NC21 engine to fit ot my NC24.

The differences in the looms that i can see are as follows:

NC21 ignition box has 3 extra wires:
Tachometer - 1 wire (NC24 tachometer drive uses different wire)
2 wires to Rocker cover - 2 wires (not present on NC24)


I guess it could be worth risking running the NC21 engine on my NC24 loom (person i bought engine off said he used the NC21 loom with a NC24 engine)